In a semiconductor process, chemical vapor deposition (CVD) processing is performed while heating an object to be processed, such as a silicon substrate or an LCD substrate, in many cases. It is general that heating of an object to be processed is performed by placing the object to be processed on a placement stage in which a resistance heater (linear or coil-shaped heater) is incorporated. That is, the object to be processed placed on the placement stage is heated by heating the placement stage itself from inside by the heater.
For example, in a CVD system, in order to prevent corrosion due to a reactant gas, and in order to improve temperature uniformity of a heating element, a placement stage is formed by a ceramics material, such as aluminum nitride (AlN), and a structure in which a resistance heating heater is embedded therein is used. When heating the object to be processed by heating the placement stage itself, in order to maintain in-plane uniformity of temperature of the placement stage, a plurality of heaters may be incorporated in the placement stage so as to control heating by each of the heaters in accordance with a temperature distribution (for example, refer to Patent Document 1).
Moreover, for example, in a plasma CVD apparatus which applies a process using plasma generated by using a radio-frequency, in order to generate plasma above a placement stage, an electrode for applying a radio-frequency is incorporated in the ceramic-made placement stage together with a resistance heating heater (for example, refer to Patent Document 2).    Patent Document 1: Japanese Laid-Open Patent Application No. 2003-500827    Patent Document 2: Japanese Laid-Open Patent Application No. 11-74064
In the placement stage in which a resistance heating heater is embedded in a ceramic material, there may be a problem in that an excessive internal stress is generated in the ceramic material due to a temperature difference according to heating by the heater and cracking occurs in the ceramic material.
For example, in order to maintain in-plane uniformity of temperature of a placement stage, the placement stage may be divided into a plurality of areas in a radial direction and a heater is provided independently in each area so as to perform a control and adjustment of the temperature of the placement surface of the placement stage. FIG. 1 is a plan view showing arrangement of heaters when the heaters are independently provided to each of divided two areas, an inner area and an outer area. Additionally, FIG. 2 is a plan view showing a part of the placement stage shown in FIG. 1 in enlargement.
The placement stage 2 is a disk-shaped ceramic material having a circular surface (placement surface 2a) matching an object to be processed, such as a wafer, and a heater 4 embedded in the interior. The placement stage 2 is divided into a plural number in a radial direction (in FIG. 1, divided into two, an outer area and an inner area), and a heater is independently provided to each area.
Both ends of a heater 4a provided in the outer area are located almost at the center of the placement stage 2, and the heater 4a extends outward in a radial direction and then enters the outer area and extends along a plurality of circumferences by being folded in the outer area. Also, both ends of a heater 4b are located almost at the center of the placement stage 2 and extends along a plurality of circumferences by being folded in the inner area. It should be noted that although the heater is drawn as a linear line in FIG. 1, it is actually a densely-wound coil form.
When a wafer, which is an object to be processed, is placed on the placement stage 2 such as shown in FIG. 1 and is heated, there is a tendency that the outer area is lower in temperature than the inner area. This is for the reason that heat goes away from the inner side to the outer side (a lower temperature side). Accordingly, there may be a case where the outer area is heated stronger than the inner area. In such as case, a temperature difference is generated between the inner area and the outer area of the placement stage 2, which generates a temperature slope near the boundary (indicated by a single-dashed chain line). When this temperature slope becomes larger than an allowable temperature slope of the ceramic material, which is the material of placement stage 2, an excessive internal stress (a thermal stress due to a temperature difference) is generated in the ceramic material, which may cause a crack generated in the placement stage with a minute defect formed during processing of a ceramic material. Ceramic materials have a small coefficient of thermal expansion but particularly sensitive to a temperature difference, and cracking of a placement stage formed by a ceramic material in which a heater is incorporated is a problem to be solved.
Moreover, when the placement stage as mentioned above is used for a processing apparatus according to plasma, there is a case where an electrode is embedded in the placement stage together with a heater. The electrode is for applying a radio frequency to a space above the placement stage, and it is general that the electrode is provided in an upper side of the heater (a side closer to the placement surface) inside the placement stage. As the material for forming the electrode, when, for example, the placement stage is formed by a ceramic material such as aluminum nitride (AlN), there is used in many cases molybdenum (Mo) which has a coefficient of thermal expansion close to that of AlN. The electrode may be in a disk-shape or a metal foil form, and may also be in a mesh-shape so as to be easily embedded in the ceramic material.
FIG. 4 is a cross-sectional view showing an outline structure of a conventional placement stage 2A in which an electrode 6 is embedded. A support member 8 for supporting the placement stage 2A and fixing to a processing container is attached at a central portion of the backside of the placement stage 2A.
In the placement stage 2A, a portion where the metal-made electrode 6 is embedded in the ceramic material is not easily deformed since the ceramic material is reinforced by the electrode 6. Here, supposing that a temperature of the front surface side of the placement stage 2A is set to T1 and a temperature of the back surface side is T2, there is a case where T2 is greater than T1 (T1<T2). For example, the back surface of the placement stage faces a bottom surface of the process chamber, and when a radiation heat emitted from the back surface of the placement stage is reflected by the bottom surface of the process chamber and returns to the back surface of the placement stage, T2 is larger than T1. In such a case, an internal stress is generated in the placement stage due to a temperature difference between the front and back surfaces of the placement stage 2A such that it is bowed and deformed as shown in FIG. 5. There is a problem in that cracking occurs near the central portion (near the portion where the support member is attached) of the placement stage due to such an internal stress.